Carbonyls & Carboxylic Acids

Which observation distinguishes propanal from propanone when each is warmed with Fehling's solution?

• ABoth give a silver mirror
• BBoth decolourise the blue solution to colourless
• CPropanal gives a brick-red precipitate; propanone shows no change✓
• DPropanone gives a brick-red precipitate; propanal shows no change

Answer: Fehling's solution oxidises aldehydes only. Propanal (an aldehyde) reduces the blue Cu²⁺ complex to a brick-red precipitate of copper(I) oxide, Cu₂O; propanone (a ketone) cannot be oxidised so the solution stays blue.

An aldehyde RCHO is oxidised to a carboxylic acid using acidified potassium dichromate(VI). What is the colour change observed?

• AColourless to pink
• BOrange to green✓
• CPurple to colourless
• DGreen to orange

Answer: Dichromate(VI), Cr₂O₇²⁻, is orange and is reduced to Cr³⁺ which is green. The aldehyde is oxidised to a carboxylic acid: RCHO + [O] → RCOOH.

Butanone is reduced using NaBH₄ in aqueous ethanol. What is the organic product, and how is NaBH₄ best described as a reducing agent here?

• AButan-1-ol; NaBH₄ delivers a hydride ion (H⁻) nucleophile
• BButan-2-ol; NaBH₄ delivers a hydride ion (H⁻) nucleophile✓
• CButan-2-ol; NaBH₄ delivers a proton (H⁺) electrophile
• DButanal; NaBH₄ delivers a hydrogen radical

Answer: NaBH₄ provides a hydride ion (H⁻) which acts as a nucleophile, attacking the carbonyl carbon. Reduction of the ketone butanone (CH₃COCH₂CH₃) gives the secondary alcohol butan-2-ol.

Ethanal reacts with HCN in the presence of a trace of KCN to form 2-hydroxypropanenitrile. What is the role of the cyanide ion, and why is a trace of KCN needed?

• ACN⁻ is the electrophile that attacks the carbonyl oxygen; the KCN provides the H⁺ ions needed to start the reaction off
• BCN⁻ removes the oxygen atom from the carbonyl group; the KCN acts as the oxidising agent that drives this step forward
• CCN⁻ is a catalyst that lowers the activation energy without taking part; the HCN itself attacks the carbonyl carbon as the nucleophile
• DCN⁻ is the nucleophile that attacks the carbonyl carbon; KCN raises the concentration of the CN⁻ nucleophile✓

Answer: The mechanism is nucleophilic addition. The cyanide ion CN⁻ attacks the δ+ carbonyl carbon. HCN is a weak acid (low [CN⁻]), so a trace of KCN increases the concentration of the CN⁻ nucleophile and speeds the reaction.

Tollens' reagent is the ammoniacal silver nitrate solution. Which species is the active oxidising agent, and what is the positive result with an aldehyde?

• AAg⁺ (as [Ag(NH₃)₂]⁺); a silver mirror forms✓
• BNH₃ (as the free base); a white precipitate of silver hydroxide forms
• CNO₃⁻; a brown gas is evolved
• DCu²⁺ (as the tartrate complex); a brick-red precipitate forms

Answer: Tollens' reagent contains the complex [Ag(NH₃)₂]⁺. An aldehyde reduces the silver(I) to metallic silver, depositing a silver mirror on the tube wall, while the aldehyde is oxidised to a carboxylate ion.

Which compound gives a positive (yellow precipitate) result in the triiodomethane (iodoform) test with alkaline aqueous iodine?

• APropanal, CH₃CH₂CHO
• BPentan-3-one, CH₃CH₂COCH₂CH₃
• CPropan-2-ol, CH₃CH(OH)CH₃✓
• DMethanol, CH₃OH

Answer: The iodoform test is positive for compounds containing a CH₃CO– group OR a CH₃CH(OH)– group (which is oxidised in situ to CH₃CO–). Propan-2-ol contains CH₃CH(OH)– and gives a pale-yellow precipitate of CHI₃. Pentan-3-one and propanal lack the required CH₃CO– group.

Why is chloroethanoic acid (ClCH₂COOH, Ka = 1.3 × 10⁻³) a stronger acid than ethanoic acid (CH₃COOH, Ka = 1.7 × 10⁻⁵)?

• AChlorine donates electron density into the carboxyl group, strengthening the O–H bond
• BChloroethanoic acid forms more hydrogen bonds with the water molecules
• CThe electron-withdrawing chlorine stabilises the carboxylate anion by delocalising its negative charge, favouring dissociation✓
• DChlorine increases the molar mass of the acid, which raises Ka

Answer: The electronegative Cl atom withdraws electron density (inductive –I effect), which disperses and stabilises the negative charge on the ClCH₂COO⁻ anion. The more stable the conjugate base, the more the equilibrium favours dissociation, so Ka is larger.

What are the products when ethanoic acid reacts with solid sodium carbonate?

• ASodium ethoxide, hydrogen and carbon dioxide gas
• BSodium ethanoate, water and carbon dioxide✓
• CEthanol, water and carbon dioxide
• DSodium ethanoate and hydrogen only

Answer: Carboxylic acids are strong enough acids to liberate CO₂ from carbonates: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. Effervescence of CO₂ is a test that distinguishes carboxylic acids from phenols (phenols do not react with carbonates).

Ethyl ethanoate is heated under reflux with aqueous sodium hydroxide. What does this alkaline (saponification) hydrolysis produce?

• AEthanoic acid and ethanol
• BEthanoic acid and ethoxide
• CSodium ethanoate and ethanol✓
• DEthanol and water only

Answer: Alkaline hydrolysis of an ester is irreversible and gives the carboxylate salt plus the alcohol: CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH. (Acid hydrolysis would instead give the carboxylic acid CH₃COOH and the alcohol, and is reversible.)

Which combination of reactants forms the ester propyl methanoate, and what is the standard condition for this esterification?

• AMethanol + propanoic acid, warmed with a concentrated H₂SO₄ catalyst
• BMethanoic acid + propanal, concentrated H₂SO₄ catalyst
• CEthanoic acid + propan-1-ol, NaOH catalyst
• DMethanoic acid + propan-1-ol, concentrated H₂SO₄ catalyst✓

Answer: An ester R-COO-R' is named acyl-from-acid + alkyl-from-alcohol. Propyl methanoate (HCOOCH₂CH₂CH₃) comes from methanoic acid (HCOOH, the acyl part) and propan-1-ol (the propyl part), warmed with a concentrated sulfuric acid catalyst.

Ethanoyl chloride (CH₃COCl) is added to water. What are the products and the key observation?

• AEthanol and chlorine; a pale green gas is seen
• BEthanoic acid and HCl; steamy/misty white fumes of HCl are seen✓
• CEthanoic anhydride and water; no visible change occurs
• DEthanal and HCl; a bright silver mirror forms

Answer: Acyl chlorides react vigorously with water: CH₃COCl + H₂O → CH₃COOH + HCl. The HCl produced is seen as steamy/misty white fumes, a characteristic test for an acyl chloride.

Ethanoic anhydride [(CH₃CO)₂O] is used industrially in preference to ethanoyl chloride to make aspirin and other esters/amides. For its reaction with an amine RNH₂, which product and advantage are correct?

• AIt gives RNH₃⁺Cl⁻ + CH₃CHO; it avoids forming an amide and so leaves the amine free for further reaction
• BIt gives RN(COCH₃)₂ + H₂O; it is a stronger oxidising agent
• CIt gives RNHCOCH₃ + HCl; it reacts far faster than the acyl chloride does and is easier to store in a laboratory
• DIt gives RNHCOCH₃ + CH₃COOH; it is cheaper, less corrosive and does not release HCl fumes✓

Answer: An acid anhydride acylates an amine to give an N-substituted amide plus a carboxylic acid: (CH₃CO)₂O + RNH₂ → CH₃CONHR + CH₃COOH. It is preferred over the acyl chloride because it is cheaper, less corrosive/violent, less readily hydrolysed by moisture, and does not produce corrosive HCl fumes.

How can an aldehyde be distinguished from a ketone?

• AKetones give a silver mirror with Tollens' reagent
• BAldehydes decolourise bromine water immediately
• CKetones give an orange precipitate with Tollens' reagent
• DAldehydes give a silver mirror with Tollens' reagent✓

Answer: Aldehydes are readily oxidised to carboxylic acids, reducing the silver ions in Tollens' to metallic silver. Ketones cannot be oxidised without breaking a carbon-carbon bond, so they give no reaction.

Why do carbonyl compounds undergo nucleophilic addition?

• AThe carbon of the polar C=O bond carries a partial negative charge
• BThe carbon of the polar C=O bond carries a partial positive charge✓
• CThe C=O bond is an area of unusually high electron density
• DThe oxygen atom is attacked by incoming nucleophiles

Answer: Oxygen is much more electronegative, so it draws electron density away and leaves the carbon electron-deficient and open to nucleophilic attack. That is the opposite of the alkene C=C, which attracts electrophiles.

What is produced when a ketone is reduced by sodium borohydride?

• AA primary alcohol
• BA tertiary alcohol
• CA secondary alcohol✓
• DA carboxylic acid

Answer: The hydride ion attacks the carbonyl carbon, and the alkyl groups on either side mean the product is a secondary alcohol. Aldehydes reduce to primary alcohols by the same mechanism.

Why does the reaction of HCN with an unsymmetrical ketone produce a racemic mixture?

• AThe nucleophile attacks the planar carbonyl from either side equally✓
• BThe nucleophile attacks the planar carbonyl group from one side only
• CThe product formed contains no chiral centre anywhere in the molecule
• DThe starting ketone is itself a racemic mixture

Answer: The carbonyl carbon is trigonal planar, so cyanide attacks from above or below with equal probability, generating both enantiomers in equal amounts. The mixture is therefore optically inactive.

What is formed when a carboxylic acid reacts with an alcohol under acid catalysis?

• AAn ester and water✓
• BAn ester and hydrogen
• CAn acyl chloride and water
• DAn amide and water

Answer: Esterification is reversible and needs concentrated sulfuric acid as catalyst. Because it is an equilibrium the yield is limited, which is why acyl chlorides are preferred when a high yield matters.

Why are carboxylic acids more acidic than alcohols?

• AThe alkoxide ion is stabilised by extensive delocalisation
• BThe carboxylate ion is stabilised by delocalisation✓
• CCarboxylic acids contain two oxygen atoms rather than one
• DCarboxylic acids are far more soluble in water

Answer: The negative charge is spread over both oxygens in the carboxylate ion, making it much more stable and the proton easier to lose. An alkoxide holds the charge on one oxygen alone.

Why does an acyl chloride react far more vigorously than a carboxylic acid?

• AHydroxide is a much better leaving group than chloride
• BThe acyl chloride carbon carries a negative charge
• CChloride is a much better leaving group than hydroxide✓
• DAcyl chlorides contain a carbon-carbon double bond

Answer: The carbonyl carbon is highly electron-deficient and chloride departs readily, so acyl chlorides react rapidly with water, alcohols and amines. That reactivity is why they are stored away from moisture.

What observation confirms a carboxylic acid when sodium carbonate is added?

• AA dense white precipitate forms in the solution
• BEffervescence, as carbon dioxide is released✓
• CThe solution turns from orange through to green
• DA silver mirror forms on the tube walls

Answer: Carboxylic acids are strong enough to displace carbon dioxide from carbonates, which phenols are not — a useful distinction between the two. The gas turns limewater milky.

What is the product of hydrolysing an ester with hot aqueous sodium hydroxide?

• AA carboxylate salt and an alcohol✓
• BA carboxylic acid and an alcohol
• CA carboxylate salt and an aldehyde
• DA carboxylic acid and an alkene

Answer: Alkaline hydrolysis goes to completion because the carboxylate salt formed cannot react back with the alcohol. Acid hydrolysis gives the acid itself but is reversible and so incomplete.